This invention relates to a polymer blend comprising a polyolefin such as polyethylene or polypropylene as the major component, a styrene polymer as a secondary component and a suitable compatibilizer therefor. More particularly, the invention relates to polymer blends which have a combination of useful properties including thermoformability, resistance to fats, ductility, low temperature impact strength, printability, relatively low cost and other characteristics which make them attractive materials for use in many applications such as packaging, plastic dinnerware, non-packaging containers, refrigerator liners, etc. The blends may be shaped into such final products by any of the known thermoplastic forming techniques such as injection molding, thermoforming, stamping, forging, solid phase forming, rotary molding or the like.
Processes for shaping or molding plastic sheets into various useful articles by forming heated thermoplastic materials over cavities and having the sheet take the shape of the cavity have been known for some years. Most commercially available machinery for such processes is designed in such a way that the thermoplastic sheet is fed through an oven and heated to bring it to a thermoformable temperature. The sheet is heated to a softening point and then advanced to a forming station. Other machines have been developed which condition the sheet directly from the extruder to the proper thermoforming temperature by means of a series of rolls, which can either be heated or cooled. In either case, the forming station is outfitted with molds of the desired shape, and the heated thermoplastic material is either draped over the male mold or formed into the female cavity, thus taking on the shape of the predetermined design. Depending upon the detail of the part being made, the forming machine can be equipped with processing assists such as vacuum and/or air pressure and/or plug assists and/or match molds.
Among the low-cost thermoforming plastic resins, polystyrene (usually impact-modified polystyrene) is the most commonly used resin because it is easy to process. Impact polystyrene grades also exhibit good impact strength and low temperature qualities. In most commercial thermoforming units, the polystyrene sheet is heated outside of the thermoforming station to its softening or melting point and then advanced into the station for the thermoforming process. Styrene polymers, in contrast with the olefin polymers, have a wide range of thermoformable temperatures and can easily be brought up to the thermoforming temperature without appreciable sag and then advanced to a thermoforming station in a relatively stable condition.
However, by contrast, olefin polymers such as polyethylene, polypropylene or ethylene-propylene copolymers have a relatively sharp melting point and, thus, when heated outside of the forming station to a thermoformable temperature, a sheet thereof will not support its own weight and will sag, making it extremely difficult to advance the sheet into the thermoforming station. Therefore, thermoforming of materials such as polypropylene or polyethylene is sufficiently difficult that it is not practical on conventional equipment.
Nevertheless, olefin polymers such as polypropylene and polyethylene have many attractive properties and, hence, it would be a great advantage to be able to process these polymers on conventional thermoforming equipment. The cost advantages of thermoforming over injection molding are well known to those experienced in polymer processing. Therefore, certain processes have been developed in the art wherein polypropylene is heated to a temperature below its crystalline melting point, i.e., the sheet is heated to a temperature below the point where it will sag, and then it is transferred into the thermoforming press or station. The forming press thus contains a sheet of polypropylene that has been partially heated to a temperature below its softening point and, as a result, is somewhat stiff for conventional forming apparatus. Hence, it is necessary to use a press which exerts considerable pressure so as to be able to form the polypropylene sheet into the desired detail part since it has not been heated to its normal thermoforming temperature. This is known as the solid phase forming process.
Other proposals have been advanced in the art for the use of certain expensive, specialized machinery designed to thermoform olefin polymers in view of their sharp melting point, such as rotary machines.
The present invention takes into account all of these problems and provides a polymer blend or alloy which can be readily formed on conventional machinery. That is, the blends of the invention incorporate the basic beneficial properties of olefin polymers such as chemical and fat resistance, high temperature properties (in the case of polypropylene), room temperature ductility and relatively low cost with the desirable properties of impact styrene polymers such as impact strength, low temperature properties, thermoformability and stiffness. The major component in the polymer blends of the present invention is the olefin polymer, whereas most of the prior art teachings heretofore concerned the addition of a polyolefin to a styrene polymer base material in order to modify its properties.
Exemplary of such prior art is U.S. Pat. No. 4,031,166 of Bronstert et al, which describes an impact-resistant thermoplastic having a major portion (70-99% by weight) of a styrenic polymer, and a minor portion (from 1 to 30% by weight) of an alpha-olefin polymer, preferably polyethylene, but also including polypropylene, polyisobutylene and copolymers of two or more alpha-olefins, together with copolymers of vinyl aromatic and diene hydrocarbons. The result is a product which has the essential characteristics of polystyrene, but which also has some of the advantages of the other constituent material.
U.S. Pat. No. 3,810,957 of Lunk discloses a molding composition of high impact strength and good oxidative stability formed by physically blending 75 to 85 weight percent of a styrenic polymer and from 30 to 15 weight percent of a selectively hydrogenated block diene copolymer. U.S. Pat. No. 3,459,830 describes a polymeric composition having improved resistance to oxidation, weathering, ozone and solvents, and comprising a major portion (100 parts by weight) of a polystyrene-polybutadiene-polystyrene block copolymer and a minor portion (20 to 40 parts by weight) of polyethylene. In each of these compositions, the styrenic polymer is the dominant material.